Atomic flame atomisation

Cadmium, copper and zinc associated with various proteins have been studied by means of an ion chromatograph coupled to a flame AAS (Ebdon et al., 1987). The design of the interface meant that the nebuliser of the AAS could be eliminated, thus avoiding the low efficiency of the nebulisation. Effluent from the HPLC was collected as discrete aliquots on a series of rotating platinum spirals that entered the flame atomiser. An atom trap (tube in flame) was included to increase the sensitivity of the detector by allowing the analyte to remain for a longer period in the optical path. 

At room temperature and at the temperatures obtained by most flame atomisers the number of atoms in the excited state is a very small fraction of those in the ground state and for absorption purposes can be ignored. We will discuss this relationship later. 

Two basic types of flame atomising systems have been used for atomic absorption. Firstly, the total consumption or turbulent burner system in which the total sample aerosol in the oxidant stream and the fuel gas are fed separately through concentric tubes to the burner jet, where the flame is burned. Considerable turbulence, both optical and acoustic, takes place. On the positive side these burner systems are very simple in construction and thus were cheap to manufacture, did not flash-back and could handle virtually any mixture of gases. However, this system is now obsolete. 

Atomic absorption spectrometry has been applied to the analysis of over sixty elements. The technique combines speed, simplicity and versatility and has been applied to a very wide range of non-ferrous metal analyses. This review presents a cross section of applications. For the majority of applications flame atomisation is employed but where sensitivity is inadequate using direct aspiration of the sample solution a number of methods using a preconcentration stage have been described. Non-flame atomisation methods have been extensively applied to the analysis of ultra-trace levels of impurities in non-ferrous metals. The application of electrothermal atomisation, particularly to nickel-based alloys has enabled the determination of sub-part per million levels of impurities to be carried out in a fraction of the time required for the chemical separation and flame atomisation techniques. 

Atomic absorption spectrophotometry (AAS) is one of the most widely used methods. Either flame atomisation or electrothermal atomisation (ETA) using the graphite furnace is employed. Some useful references are given in the Bibliography. 

The following colorimehic and atomic absorption spectropbotomehic mediods are applicable to serum. The atomic absorption method uses a conventional air-acetylene flame atomiser. 

Numerous procedures for die determination of lead in blood by atomic absorption spectrophotometry have been reported. The first mediod described here uses a nickel cup to contain the sample, a conventional air-acetylene flame atomiser, and a... 

Flame atomisation is not necessary for the atomic absorption spectrophotomehy of mercury. The cold vapour technique described here employs a reduction vessel (which may be purchased) to produce mercury vapour the vapour is led to a quartz absorption cell within the atomic absorption inshument. The method is applicable to inorganic and organic mercurial compounds in urine. 

The following atomic absorption spectrophoto-metric method is applicable to blood and urine, and uses a conventional air-acetylene flame atomiser. Standard Thalliim Solutions. Dissolve 1.3034g of thallous nitrate (TINO3) in sufficient water to produce 1000 ml. This solution contains 1 mg ofTl in 1 ml. Serially dilute the solution with water to produce solutions containing 0.2, 0.5, 1.0, 2.0, and 4.0 p,g of T1 in 1 ml. 

Alvarado, J. R. Jaffe, 1998. Tube in flame atomisation away ofenhancing detection limits in flame atomic absorption spectrometry. J. Analytical Atomic Spectrom. 13 37-40. 

Flame atomisation In flame atomisation, the sample solution is introduced into the flame with a particularly designed nebuHser (Fig. 12.6). The function of the nebuHser is to disrupt the continuous sample stream into a mist of fine droplets of typically 5—20 pm diameter which are swept into the mixing chamber. The aerosol is then mixed with the fuel gas and the oxidant gas before reaching the burner head. As a number of physical and chemical reactions, e. g., vaporisation, dissociation, reduction, or oxidation, may occur, it becomes evident that precise control of the operating conditions of flame atomisation is required to obtain stable and sensitive signals. Optimisation of flame atomic absorption measurements has thus the double role of maximising the elemenf s response while minimising the undesired side-reactions. 

The determination of the different forms (e. g. compounds or complexes) in which an element occurs (often referred to as the speciation of an element and speciation analysis, respectively [28]) is in most cases performed by hyphenated techniques. These are the combination of a high-performance separation technique such as gas or liquid chromatography, or electrophoresis, and an element- or compound-specific detector [29]. While the former provides the separation of the different elemental species prevalent in the sample, the latter brings selective and sensitive detection. In the case of AAS, only the hyphenation with gas and Hquid chromatography, respectively, has gained importance. The combination of atomic absorption spectrometry and electrophoresis has never proven successfiil, obviously due to the incompatibility of the extremely low flow rates of electrophoretic separations with the aspiration volumes of flame atomisers and the difficulties of interfacing the two techniques. 

The sensitivity difference between direct flame atomisation and furnace atomisation has been bridged via the general method of atom trapping as proposed by Watling [37]. A silica tube is suspended in the air-acetylene flame. This increases the residence time of the atoms within the tube and therefore within the measurement system. Further devices such as water-cooled systems that trap the atom population on cool surfaces and then subsequently release them hy temporarily halting the coolant flow are sometimes employed. The application of atom-trapping AAS for the determination of lead and cadmium has been discussed by Hallam and Thompson [38]. 

Trace elements Solutions obtained after digesting samples of about 0.2 g were analysed using either Atomic Absorption Spectrometry (AAS) with graphite furnace atomisation (Perkin-Elmer 4000, HGA 500), or flame atomisation (Varian Techtron), or... 

Atomic absorption spectroscopy involves atomising the specimen, often by spraying a solution of the sample into a flame, and then studying the absorption of radiation from an electric lamp producing the spectrum of the element to be determined. 

Although electrothermal atomisation methods can be applied to the determination of arsenic, antimony, and selenium, the alternative approach of hydride generation is often preferred. Compounds of the above three elements may be converted to their volatile hydrides by the use of sodium borohydride as reducing agent. The hydride can then be dissociated into an atomic vapour by the relatively moderate temperatures of an argon-hydrogen flame. 

Both emission and absorption spectra are affected in a complex way by variations in atomisation temperature. The means of excitation contributes to the complexity of the spectra. Thermal excitation by flames (1500-3000 K) only results in a limited number of lines and simple spectra. Higher temperatures increase the total atom population of the flame, and thus the sensitivity. With certain elements, however, the increase in atom population is more than offset by the loss of atoms as a result of ionisation. Temperature also determines the relative number of excited and unexcited atoms in a source. The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 103 to 1010 or more. At higher temperatures (up to 10 000 K), in plasmas and electrical discharges, more complex spectra result, owing to the excitation to more and higher levels, and contributions of ionised species. On the other hand, atomic absorption and atomic fluorescence spectrometry, which require excitation by absorption of UV/VIS radiation, mainly involve resonance transitions, and result in very simple spectra. 

Principles and Characteristics Flame emission instruments are similar to flame absorption instruments, except that the flame is the excitation source. Many modem instruments are adaptable for either emission or absorption measurements. Graphite furnaces are in use as excitation sources for AES, giving rise to a technique called electrothermal atomisation atomic emission spectrometry (ETA AES) or graphite furnace atomic emission spectrometry (GFAES). In flame emission spectrometry, the same kind of interferences are encountered as in atomic absorption methods. As flame emission spectra are simple, interferences between overlapping lines occur only occasionally. 

In AFS, the analyte is introduced into an atomiser (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (HCL, EDL, or tuned laser). Subsequently, the fluorescence radiation is measured. In the past, AFS has been used for elemental analysis. It has better sensitivity than many atomic absorption techniques, and offers a substantially longer linear range. However, despite these advantages, it has not gained the widespread usage of atomic absorption or emission techniques. The problem in AFS has been to obtain a... 

A flame, where the solution of the sample is aspirated. Typically, in FAAS the liquid sample is first converted into a fine spray or mist (this step is called nebulisation). Then, the spray reaches the atomiser (flame) where desolvation, volatilisation and dissociation take place to produce gaseous free atoms. Most common flames are composed of acetylene-air, with a temperature of 2100-2400 °C, and acetylene-nitrous oxide, with a temperature of 2600-2900 °C. 

Another type of interference that can arise in the atomiser is called ionisation interferences . Particularly when using hot atomisers, the loss of an electron from the neutral atom in metals with low ionisation energy may occur, thus reducing the free atom population (hence the sensitivity of the analyte determination, for which an atomic line is used, is reduced). These interferences can be suppressed in flames by adding a so-called ionisation suppressor to the sample solution. This consists in adding another element which provides a great excess of electrons in the flame (he. another easily ionisable element). In this way, the ionisation equilibrium is forced to the recombination of the ion with the electron to form the metal atom. Well-known examples of such buffering compounds are salts of Cs and La widely used in the determination of Na, K and Ca by FAAS or flame OES. 

It is possible to extend the principle of photometric emission (FPD) by replacing the flame with a microwave plasma that has a temperature high enough to induce any element to radiate light. This is equivalent to atomic emission where each solute is atomised and gives rise to specific emission bands. 

The thermal device used to elevate the temperature consists of a burner fed with a gaseous combustible mixture or, alternatively, in atomic absorption, by a small electric oven that contains a graphite rod resistor heated by the Joule effect. In the former, an aqueous solution of the sample is nebulised into the flame where atomisation takes place. In the latter, the sample is deposited on the graphite rod. In both methods, the atomic gas generated is located in the optical path of the instrument. 

Figure 14.16—Elements determined by AAS or FES. Most elements can be determined by atomic-absorption or flame emission using one of the available atomisation modes (burner, graphite furnace or hydride formation). Sensitivity varies enormously from one element to another. The representation above shows the elements in their periodic classification in order to show the wide use of these methods. Some of the lighter elements, C, N, O, F, etc. in the figure can be determined using a high temperature thermal source a plasma torch, in association with a spcctropholometric device (ICP-AbS) or a mass spectrometer (1CP-MS).

To avoid problems previously encountered with flame atomic absorption spectrometry of arsenic, and also with flameless methods such as that in which the dementis converted to arsine, Ohta and Suzuki [25] proposed an alternative method based on electrothermal ionisation with a metal microtube atomiser. Effective atomisation can be achieved by the addition of thiourea to the arsenic solution or by preliminary extraction of the arsenic-thionalide complex. The second method is recommended for soil samples so as to avoid interference due to the presence of trace elements. 

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